1. Field of the Invention
The present invention relates to a method, and an apparatus for the calibration of stock consistency sensors that serve to measure the stock consistencies of fiber stock suspensions in the production of a fiber web, specifically a paper or cardboard web.
2. Description of the Related Art
In the stock supply system that is in the approach section of a paper production line the measurement of the stock consistency is one of the most important parameters in process control. Consequently, concentrations of fiber stock suspensions, that is suspensions having a high stock consistency expressed as a concentration in g/l of fiber stock, as well as suspensions having a low stock consistency, for example wire water I and wire water II, are measured, and sensors located accordingly. Consistent with the principles of measurement, the measurement of a suspension is difficult and results can vary due to changes in the stock composition or the type of filler. A particular disadvantage is that the sensors have to be verified and re-calibrated off-line; in other words when they are removed from the line, or through random sampling with a more favorable stock consistency in a laboratory. This is time and cost intensive. The calibration establishes the relevant connection between the stock consistency value provided by the measuring device and the actual stock consistency.
What is needed in the art are stock sensors, which can be easily and economically calibrated.
The present invention provides a method and apparatus to calibrate stock consistency sensors, which serve to measure the stock consistencies of fiber stock suspensions in the production of a fiber web, specifically a paper or cardboard web. For this purpose stock consistency sensors, all based on the same measuring principle, are used and these stock consistency sensors are calibrated on-line and automatically. The actual stock consistency values are determined from the obtained stock consistency readings by way of at least one mass balance and by utilizing the characteristic curves of the sensors.
This arrangement facilitates the calibration of stock consistency sensors on-line and automatically. The measured stock consistencies are verified and corrected continuously, thereby rendering the paper manufacturing process more stable and efficient. Since associated operational and qualitative parameters, specifically stock consistencies, retention, longitudinal profiles of basis weights, formation, etc. are subject to fewer fluctuations and are, therefore, almost constant, the result is increased product quality. The automatic calibration of each consistency sensor or transmitter results in a more precise process control, as well as in cost savings.
In accordance with a preferred embodiment of a method of the present invention a stock consistency sensor is utilized, that supplies a measured stock consistency value, which coincides with the relevant actual stock consistency value, whereby this directly measured stock consistency value is applied during the construction of the mass balance equation. At least a part of the volume flows entering into, and emerging from, the balance enclosure are measured and the measured values are utilized in constructing the mass balance equation.
The stock consistency sensors, that are to be calibrated, can be located specifically in the approach flow section that is in the stock supply section, of the production line. The stock consistency sensors, that are to be calibrated, may for example include sensors having linear characteristic curves. The stock consistency sensors, that are to be calibrated, are preferably assigned to pipes, through which the fiber stock, or stock having the same stock characteristics, flows.
According to a functional embodiment of the method of the present invention, the stock consistency sensors are always assigned to a pipeline carrying thick stock. Therefore, a mass balance may be constructed for a partial section of the production line to which thick stock and dilution water are supplied and from which the diluted thick stock is discharged. The stock consistency sensors, that are to be calibrated, measure the stock consistencies of the supplied undiluted thick stock and the discharged diluted thick stock.
In certain instances, it is also an advantage to incorporate into a mass balance equation the measured surface mass, down stream from the paper machine; and/or the dryer section; and/or down stream from the press; and/or the measured dry content following the press.
If the entire system of the wet-end process and the relevant control architecture are viewed together, as a self-contained model, then the precisely functioning calibrating sensor at the end of the paper machine may, for example, check and if necessary reset the suspension sensors in the approach flow section either on-line or periodically. This applies equally to the overall solids consistency, as well as the filler concentration, since both values may be captured by way of the suspension sensors and the traversing scanner.
The apparatus according to the present invention, for calibrating stock consistency sensors that serve to measure the stock consistencies of fiber stock suspensions in the production of a fiber web, specifically a paper or cardboard web, include stock consistency sensors. Each stock consistency sensor is based on the same measuring principle, in order to calibrate the stock consistency sensors on-line and automatically. The actual stock consistency values are determined from the obtained stock consistency readings by way of at least one mass balance equation and by way of the characteristic curves of the sensors.